This invention relates to methods and apparatus for measuring cathode emissions of vacuum tubes and more particularly for measuring cathode emission slump in cathode ray tubes.
Apparatus for measuring cathode emission is not new. For example, an apparatus for measuring nominal emission currents of x-ray tubes is disclosed in U.S. Pat. No. 3,878,455 issued to Ochmann. Also, an apparatus for measuring peak emission of cathodes in color television picture tubes is disclosed in U.S. Pat. No. 3,654,548 issued to Pecorari et al. Further, an apparatus for measuring the beam current component of cathode emission in television picture tubes is disclosed in U.S. Pat. No. 2,732,526 issued to Futterman.
Although the above-referenced apparatus are useful in measuring cathode emission at a particular point in time, it has been found that cathode emission will most likely vary as a function of time. It has been further determined that an important parameter in determining future picture tube performance is cathode emission slump which is defined as a decrease in the magnitude of cathode emission over a period of time.
It can be empirically determined that if the difference between the peak emission current and the lowest value of emission current measured during a predetermined time period is greater than a predetermined magnitude, the tube will be susceptible to premature cathode failures. Consequently, in the mass production of quality television picture tubes, it is highly desirable that the emission slump test be automatically performed on every tube in order to cull out the potentially weak performers.